Protective Layers for Metal Electrode Batteries

ABSTRACT

Rechargeable metal batteries are disclosed having a protective ionic membrane layer on a metal anode. The ionic membrane can be formed from ionomers in the electrolyte including polymerizable ionic liquid monomers or halogenated alkyl anion salts. Such ionic membranes can continuously supply ions near the anode electrode and stabilize the anode metal electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos.62/436,248, filed 19 Dec. 2016, 62/556,037 filed 8 Sep. 2017, and62/572,943, filed 16 Oct. 2017, the entire disclosures of which arehereby incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant numbersDMR-1609125 and DMR-1120296 awarded by the National Science Foundationand grant numbers DE-AR0000750 and DE-FOA-001002-2265 awarded by theDepartment of Energy. The government has certain rights in theinvention.

TECHNICAL FIELD

The present disclosure relates to protective layers for metal electrodebatteries, methods of making the protective layers, and batteriescomprising the protective layers.

BACKGROUND

High energy rechargeable batteries based on active metal (Li, Na, Al,Si, Sn, Zn, etc.) anodes are among the most important electrochemicalenergy storage devices to supply power for rapidly evolvingtechnologies, including the fields of portable electronics, advancedrobotics, electrification of transportation, etc. It has long beenunderstood that such metal based anodes offer factors of 2-10 timeshigher specific capacity (e.g., 3860 mAh/g for Li), compared with thecarbonaceous anode (360 mAh/g) used in lithium ion battery technology.Some metal anode batteries are also advantageous because they enable thedevelopment of high-energy unlithiated materials, such as sulfur,oxygen, and carbon dioxide as the active species in the cathode. Thisraises the prospect of multiple battery platforms that offer largeimprovements in specific energy on either a volumetric or mass basis.

Despite their promise, rechargeable batteries based on lithium (Li)metal, sodium (Na) metal, and aluminum (Al) anodes are not commerciallyviable today because the metals are plagued by one or moreinstabilities. Chemical instability of the metal in contact with liquidand solid-state ceramic electrolyte depletes the electrolyte andelectrode overtime, leading to run-away increases in cell resistance. Atlow current densities, spatial heterogeneities in the conductivity ofspontaneously formed, fragile solid electrolyte interphases (SEIs) on Lilead to rough plating of the metal during battery recharge as electricfield lines concentrate on thinner, more conductive SEI that providefaster growth leading to the morphological instability typicallyassociated with mossy, high-surface-area deposits. At high currentdensities, depletion of ions from an electrochemically active surfaceleads to formation of the hydrodynamic instability termedelectroconvection, which drives selective metal deposition at localizedregions on the electrode to form diffusion limited fractal structurestermed dendrites. Because the driving force for the last of these threeinstabilities is physical, all batteries based on charge storage byreduction of metal ions at the anode (e.g. Li, Na, Al) will fail by thismechanism at high currents.

Uncontrolled growth of metallic structures created as a result ofmorphological or hydrodynamic instabilities leads to battery failure byformation of internal shorts, which limits the cell lifetime. Even ifthis failure mode can be prevented through choice of electrolyteadditives the chemical and physical fragility of the formed structurescause cell failure by other means, typically loss of active material inthe anode, which manifests as a low charge utilization or Coulombicefficiency. Additionally, because Li-ion cells based on high-energymetallic anodes including Si, Sn, and Ge store Li by alloying reactions,which produce large cyclic volume change in the electrode and destroysthe SEI formed on the electrode each cycle, the first of the threeinstabilities are common to Li-ion batteries based on any of thesechemistries.

The main hurdles preventing large-scale deployment of batteries based onmetal anodes stem from the uneven electrodeposition of metal ions duringbattery recharge and parasitic reactions between the metal anode andliquid electrolytes during all stages of battery operation. For example,batteries based on lithium or sodium are well-known to form rough,dendritic structures upon being reduced as a result of their intrinsictendency to deposit on protrusions where the electric field lines areconcentrated. Accumulated dendritic deposits can connect two electrodes,causing short-circuit and other safety-related hazards. In less extremecases, dendrites promote the parasitic reaction of metal and theelectrolyte which lowers the Coulombic efficiency and deteriorates thebattery performance over time. This can be a more severe problem in acapacity balanced ‘full cell’ in which very limited amount of the metalspecies are present, in contrast with lab-based ‘half-cell’ whereusually excessive metal anodes are used.

It has been previously demonstrated that, elements such as Si, Sn, In,Mg, and Ge are able to reversibly form alloys with lithium (e.g.,Li_(4.4)Sn), meaning these materials provide certain capacity inrechargeable lithium metal batteries. However, when used as stand-anodeanodes, Si, Sn, In, Mg, and Ge undergo large volume changes (as high as300%) upon lithiation, which destroys electrical connection with thecurrent collectors and causes premature battery failure. The effects areeven more severe for batteries based on sodium. This problem waspreviously addressed by two methods: creating composite anodes in whichthe metal is integrated with an inert metal of conductive carbon (e.g.graphene, graphite, carbon black, carbon nanotubes), or by fabricatingthe active materials (e.g. Si, Sn, In, Mg, Ge) nanostructures in variousmorphologies (e.g., nanowires, nanotubes, hollow particles) able toaccommodate the volume expansion. The first method inevitably lowers thespecific capacity as the inert metal or carbon materials used in thecomposites only serve as mechanical reinforcement for the active Si, Sn,In, Mg or Ge material and does not participate in the electrochemicalreaction. Additionally, the alloying process is typically slow, andideally requires higher temperature than those normally used inbatteries for long-term stability. Fabricating the active material inthe form of nanostructures does extend the anode lifetime, but comes atconsiderable costs in terms of the lower density of the active material,which reduces the specific energy on a volume basis of the electrode.The added costs of the processes required to create nanostructures inthe desired shapes at scale and with high degrees of reproducibility forbattery manufacturing also introduce new challenges that have so farproven to be stubborn roadblocks to commercialization of batteriesemploying metals as anodes.

Hence, there is a continuing need to develop high energy rechargeablebatteries based on active metal anodes.

SUMMARY OF THE DISCLOSURE

An advantage of the present disclosure is a battery having a protectedmetal electrode, e.g., metal anodes that comprise substantially metalliclithium, metallic sodium, metallic aluminum, metallic zinc, etc.Advantageously, the battery is a rechargeable battery that can include ametal anode, a cathode and an electrolyte, in which the metal anode hasan ionic membrane thereon. Advantageously, the ionic membrane cancontinuously supply ions near the anode electrode by providing ahalogenated salt at or near the anode-electrode interphase, for example.The ionic membrane can further protect the metal anode.

These and other advantages are satisfied, at least in part, by arechargeable battery comprising a metal anode, a cathode and anelectrolyte, wherein the metal anode has an ionic membrane thereon. Theionic membrane can be formed from ionomers including polymerizable ionicliquid monomers to form an ionic polymer membrane or from halogenatedalkyl anion salts to form alkyl anions tethered to the metal anode.

In one aspect of the present disclosure, the ionic membrane can includean ionic polymer membrane directly on the metallic anode, e.g., ametallic sodium anode. Such ionic polymer membranes can be formed fromone or more polymerizable ionomers, such as one or more polymerizableionic liquids (IL) having one or more allyl or vinyl groups and havingan anion. The polymerizable ionomers can be included with an electrolyteof a rechargeable battery.

In another aspect of the present disclosure, the ionic membrane caninclude alkyl anions tethered to the metal anode, e.g., a metalliclithium anode. Such ionic membranes can be formed from ionomers ofhalogenated alkyl anion salts, such as one or more halogenated alkylsulfonate salts. The halogenated alkyl anion salts can be included withan electrolyte of a rechargeable battery.

Advantageously, the anodes protected with the ionic membrane of thepresent can be cycled stably with high Coulombic efficiency at highcurrent densities. For example, metal anodes protected with ionicmembranes of the present disclosure can exhibit coulombic efficiencyexceeding 90% such as exceeding 95%. Such protected anodes can exhibitstable cycling of over 100 cycles such as over 150 cycles.

Other aspects of the present disclosure include processes for preparingionic membranes on metallic anode electrodes, which can be used in arechargeable battery. Such processes include electropolymerizing apolymerizable ionic liquid monomer onto the metal anode to form an ionicmembrane thereon. Other processes include adsorbing halogenated alkylanion salts onto a surface of the metal anode electrode to form an ionicmembrane thereon. The IL monomer and the halogenated alkyl anion saltscan be included with an electrolyte of the rechargeable battery. Theaddition of such ionomers in the electrolyte can help in theregeneration of the ionic membrane in repeated cycling of the battery.

Additional advantages of the present invention will become readilyapparent to those skilled in this art from the following detaileddescription, wherein only the preferred embodiment of the invention isshown and described, simply by way of illustration of the best modecontemplated of carrying out the invention. As will be realized, theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects, allwithout departing from the invention. Accordingly, the drawings anddescription are to be regarded as illustrative in nature, and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the attached drawings, wherein elements having thesame reference numeral designations represent similar elementsthroughout and wherein:

FIG. 1 is a schematic representation of forming an ionic polymermembrane on a surface of a metal anode, e.g., a sodium anode, inaccordance with an embodiment of the present disclosure.

FIG. 2 is a plot showing cell failure time at various current densitiesderived from constant current polarization test of Na/Na symmetric cellwith and without 20 wt % DAIM in the electrolyte.

FIG. 3A shows cycling performance of Na₃V₂(PO₄)₃ cells with and withoutprotected sodium metal anode at 100 mA/g of the active material in thecathode.

FIG. 3B shows cycling performance of Na-SPAN cells with and withoutprotected sodium metal anode at 0.2 C (1C=1675 mAh/g) based on sulfur inthe cathode.

FIG. 4 is a schematic representation of forming an ionic membrane on asurface of a metal anode, e.g., a lithium anode, in accordance with anembodiment of the present disclosure.

FIG. 5 is a plot comparing interfacial and bulk impedance values forionomer-based and control electrolytes as a function of time. Thecircles denote results with the control electrolyte (1 MLiNO₃—N,N-dimethylacetamide (DMA)), whereas the squares and trianglesrepresent batteries with 10 and 5% (by weight) ionomer additive,respectively, with the same electrolyte (open symbols represent bulkimpedance and solid symbols represent interfacial impedance).

FIG. 6 is a voltage profile of the Li∥SS cell plotted over time. In thisexperiment, Li⁺ ions were deposited onto the stainless steel side at acurrent density of 1 mA/cm² for 10 hours, after which the cell was keptat rest for an additional 10 hours, as shown in the current-versus-timecurve. In the voltage-versus-time graph, the red line represents theprofile of the control electrolyte (1 M LiNO₃-DMA), whereas the blackline is for the same electrolyte enriched with 10% (by weight) ionomeradditive. The dashed line in the current-versus-time graph is theapplied current for both cases.

FIG. 7 is a plot of cycle number associated with divergence of voltageagainst respective current densities for a Li∥SS cell in which lithiumwith 10-mAh/cm2 capacity is deposited onto SS, and the battery wascharged and discharged consecutively at various current densities.

FIG. 8 shows end voltage of charging cycle for the control and theionomer-added electrolyte plotted as function of cycle number.

DETAILED DESCRIPTION OF THE DISCLOSURE

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments which may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the disclosure, and it is to be understood thatother embodiments may be utilized and that structural, logical, and/orother changes may be made without departing from the scope of thepresent disclosure. The following description of example embodiments is,therefore, not to be taken in a limited sense.

Ranges of values are disclosed herein. The ranges set out a lower limitvalue and an upper limit value. Unless otherwise stated, the rangesinclude all values to the magnitude of the smallest value (either lowerlimit value or upper limit value) and ranges between the values of thestated range.

The present disclosure provides protective layers for electrodes (e.g.,anodes) of batteries. The present disclosure also provides methods ofmaking protective layers for the electrodes of batteries, and devicescomprising same. Various embodiments and examples, describe provingsafe, stable, air-insensitive metal anodes (e.g., lithium or sodiummetal anode) by protecting the surface with an ionic membrane layerthrough in situ or ex situ facile wet chemistry reaction, with noharmful byproducts generated. The ionic membrane provides desirable(e.g., low) surface impedance and protects metallic anodes fromparasitic reaction with the electrolyte. The methods disclosed hereincan be readily incorporated into large-scale manufacturing.

In this disclosure, a metallic anode is protected by an ionic membranedirectly on a surface of the anode exposed to electrolyte. The protectedanode can be part of a rechargeable battery including the metal anode, acathode and an electrolyte. The ionic membranes of the presentdisclosure can be formed from ionomers. As used herein an ionomer is acompound that has ionizable or ionic groups, or both and that can eitherpolymerize to form an ionic membrane on a surface of a metallic anode orcan adsorb on the surface of the metallic anode to form an ionicmembrane thereon. Advantageously, the ionomers form thin conformalcoatings directly on the anode surface exposed to the electrolyte. Ionicmembranes prepared from ionomers of the present disclosure cancontinuously supply ions near the anode electrode and stabilize anodemetal. In certain embodiments, the ionic membrane can have a thicknessin the range of a monolayer to micron scale, e.g., from about 5 nm toabout 500 microns.

In one aspect of the present disclosure, the ionic membrane can includean ionic polymer membrane directly on the metallic anode. Such ionicpolymer membranes can be formed from polymerizable ionomers, which canbe included with an electrolyte of a rechargeable battery. Usefulpolymerizable ionomers of the present disclosure include, for example,polymerizable ionic liquids (IL) having one or more allyl or vinylgroups and a corresponding anion. Such IL monomers include 1,3-diallylimidazolium perchlorate (DAIM), 1-allyl-3-vinyl imidazolium perchlorate(AVIM), 1,3-diallyl piperidinium hexafluorophosphate, 1,3-diallylpiperidinium bis(trifluoroethanesulfonyl)imide, 1,3-diallyl imidazoliumbis(fluorosulfonyl)imide, etc. Preferably, the IL monomer has more thanone polymerizable group and a corresponding anion.

In another aspect of the present disclosure, the ionic membrane caninclude alkyl anions tethered to the metal anode. Such ionic membranescan be formed from ionomers of halogenated alkyl anion salts, e.g.,halogenated alkyl sulfonate salt, which can be included with anelectrolyte of a rechargeable battery. Useful halogenated alkyl anionsalts of the present disclosure include, for example, those with thefollowing formula:

X-alkyl-A

wherein X represents a halogen such as a chlorine, bromine, iodine;alkyl represents a C₁₋₁₂, e.g., C₂₋₆ divalent alkyl group; and Arepresents a anion salt such as a sulfonate metal ion salt, e.g.,lithium sulfonate, where the metal ion is preferably a metal ioncorresponding to the metal of the anode. Examples of ionomers that canabsorb onto a metal anode include 2-bromoethanesulfonate lithium salt,2-bromoethanesulfonate sodium salt, 2-chloro-ethane sulfonate sodiumsalt, 2-bromoethane-bis (fluorosulfonyl) imide lithium salt. Suchhalogenated alkyl anion salts form alkyl anions tethered to the metalanode, i.e., M-alkyl-A⁻ ⁺M where M is the metallic anode electrodesurface and A⁻ ⁺M is the anion salt.

Another aspect of the present disclosure is processes for preparingionic membranes on metallic anode electrodes, which can be used in arechargeable battery. Such processes include electropolymerizing apolymerizable ionic liquid monomer onto the metal anode to form an ionicmembrane thereon. Other processes include adsorbing halogenated alkylanion salts onto a surface of the metal anode electrode to form an ionicmembrane thereon. The IL monomer and the halogenated alkyl anion saltscan be included with an electrolyte of the rechargeable battery. Theaddition of such ionomers in the electrolyte can help in theregeneration of the ionic membrane in repeated cycling of the battery.In an embodiment, the ionomer, e.g., polymerizable ionic liquid monomeror halogenated alkyl anion salts or both, can be included as an additivein the electrolyte of a rechargeable battery at concentration of about1% to about 30% by weight such as of about 5% to about 20% by weightbased on the total weight of the electrolyte.

Advantageously, the anodes protected with the ionic membrane of thepresent can be cycled stably with high Coulombic efficiency at highcurrent densities. For example, metal anodes protected with ionicmembranes of the present disclosure can exhibit coulombic efficiencyexceeding 90% such as exceeding 95%. Such protected anodes can exhibitstable cycling of over 100 cycles such as over 150 cycles.

Metal anodes that can benefit from the present disclosure include metalanodes that comprise substantially metallic lithium, metallic sodium,metallic aluminum, metallic zinc, etc. Forming an ionic membrane on thesurface of such anodes according to the present disclosure provides asolid electrolyte interface (SEI) that offers desirable (e.g., fast) iontransport between a bulk liquid electrolyte and a metal anode (e.g., Lior Na), or optionally an intercalating or conversion cathode, but at thesame time protects the electrodes from physical contact with the liquid,which provides an important route towards creation of reactive metalanodes able to overcome chemical instability of the metal anode. Thus,the advantages of a metal anode (e.g., metallic lithium or sodiumanodes) such as, for example, high energy density and low voltage, arenot compromised, while its cycle life and/or safety can be enhanced.

Various embodiments and examples of the present disclosure are providedbelow.

In one aspect of the present disclosure, an ionic membrane can beprepared as an SEI film formed directly on a metallic anode, e.g., ametallic sodium anode, through electropolymerization of a polymerizableionic liquid monomer in a liquid electrolyte. It was found that suchmembranes markedly increase the stability of anodes to failure. We alsofound that unlike Li anodes, which can fail by any of the processesdiscussed in the Background section above, Na anodes almost always failas result of electrolyte degradation. The ionic membrane of the presentdisclosure advantageously can exhibit exceptional chemical andelectrochemical stability in contact with reactive metals and in certainenvironments stabilize lithium and sodium metal anodes against dendriteformation such as with aprotic liquid electrolytes. Theoretical worksuggests that the tethered anions in an ionic liquid can improve thestability of Li plating during the battery recharge by acting as asupporting electrolyte. This has been verified by the experimentalstudies which show that dendrite free electrodeposition can be achievedby uniforming ion distribution by contacting lithium to functional glassfiber or solid polymer electrolyte. An ionic liquid additive is believedto reduce the magnitude of destabilizing electric fields near the anodeby providing a localized supply of anions able to support a high cationflux at the electrode. This effect is quite general and is not limitedto Li. Carbonate based electrolyte containing around 5 to 10 vol %1-methyl-3-propylimidazolium-chlorate tethered to SiO₂ nanoparticleswere shown to enhance the stability and cycle life for both Na—S andNa—O₂/CO₂ cells. (Nat Commun, 2016, 7, 11722 and ChemSusChem 2016, 9,1600-1606).

Secondary sodium-ion and sodium-metal batteries offer multipleadvantages over their lithium counterparts for portable storage inelectric vehicles and locomotives as well as for stationary storage inpower stations. Among the motivations for interest in these batteriesinclude the low cost and high natural abundance of sodium, the hightheoretical capacity (1166 mAh/g) and moderate redox potential (−2.71 Vversus the standard hydrogen potential) of the sodium anode, and thefact that high-temperature sodium-sulfur (Na—S) and Na-metal chloride(Na/MeCl₂) batteries have already been demonstrated to be commerciallyviable technologies for grid storage and for electrification oftransportation. On-field accidents and difficult maintenance of thesebatteries due to their high operating temperature are a concern forbroader deployment in portable devices, robotics, and electric vehicles.Room-temperature sodium batteries based on high-voltage intercalationcathodes or energetic conversion cathodes (e.g. Na—S or Na—O₂/CO₂ cells)have already been reported to overcome some of these difficulties,however the focus on improvements afforded by ambient-temperatureoperation, typically gloss over fundamental challenges associated withlong term stability of a sodium metal anode in liquid electrolytes.

Recharge of any battery utilizing a sodium metal anode requiresrepetitive stripping and platting of the metal surface. The sizedifference between sodium ions and sodium atoms subjects the anode tolarge volumetric and morphological changes during normally batterycycling, which makes the batteries unstable to failure by multipleprocesses. First, because of its high reactivity, metallic sodium rarelyforms stable solid electrolyte interphases, rather it is corrodedcontinuously by aprotic liquid electrolytes upon cycling, which lowersthe Coulombic efficiency (CE) and lifetime of the cells. Second, theuncontrolled nature of the SEI formation reactions lead to significantspatial variations in SEI conductivity, which drives spatial variationsin Na deposition during battery recharge making the metal particularlyunstable towards the morphological instability known as dendriticelectrodeposition. Finally, because of the metal's softness and lowmelting point, dendrite induced short circuits either end in thermalrun-away or in sodium dendrites breaking away from the metal electrodeto become electrochemically disconnected from the metal substrate, whichshorten the lifetime of the cell.

Ionic membranes formed directly on metallic sodium anodes can mitigatethe stability problems of such anodes. The following provides an exampleof preparing such an ionic membrane from a polymerizable ionic liquid(IL) monomer.

Polymeric Ionic Liquid Film Formation and Characterization

Electrochemical polymerization of reactive monomers on metallic surfacesis a well-established method to fabricate functional polymer thin filmsfor protecting metals and electronic devices, including sensors. Theapproach offers several advantages over other polymerization techniquesthat may also be used to create coatings on metals, including: (i) itcombine polymer synthesis with thin film formation; (ii) it eliminatesthe need for exogenous oxidants to initiate the polymerization; and(iii) properties of the membrane, including its thickness, morphology,and porosity are controlled by transport processes in the film itself.Although the method has not been previously studied to protect reactivealkali metals for battery applications, it is well suited to thisapplication because the transport processes that control film thicknessand morphology are related to those that control access of ions to/fromthe metal surface during charge/discharge processes in a battery. FIG. 1is a schematic representation of the process, showing how it can be usedto create an ionic polymer membrane on the surface of a metal anode,e.g., a sodium anode. The polymerization reaction is believed toprogress via the usual steps (initiation, propagation, and termination)that govern free radical polymerization processes. Initiation occursduring charging, wherein the unsaturated ionic liquid monomers acceptelectrons to form reactive radical species. These species react withmonomers to increase the size of the radicals, propagating thepolymerization process and creating a thin, porous membrane in conformalcontact with the anode.

The molecular structure of the ionic liquid (IL) monomer was found to bea factor in regulating the structure and morphology of the polymermembrane as well as in controlling the degree of polymerizationachieved. It was reported that polymerization of IL molecules may resultin degradation of properties of the monomers, including ionicconductivity, due to elevation of the glass transition temperature andreduced number of mobile ions after covalent bonding of the componentions. In order to achieve optimized adhesion and ionic conductivity,different functional imidazolium cation-based ionic liquid monomersbearing allyl or vinyl group and perchlorate anion, namely1-allyl-3-methylimidazolium perchlorate (AMIM), 1,3-diallyl imidazoliumperchlorate (DAIM), 1-allyl-3-vinyl imidazolium perchlorate (AVIM) weresynthesized, and their electroinitiated polymerization process on theelectrode surface was investigated. To characterize the polymeric filmformed on the electrode, the monomers were dropped on a polypropyleneseparators sandwiched by two stainless steel electrodes in a coin cell.A constant current of 1 mA/cm² was applied to the cells to initiatepolymerization until the voltage goes diverging with no current passingthrough the cell, at which point the electrodes are probably covered bythe films and completely insulated. The morphology of the polymer filmwas examined by scanning electron microscopy (SEM). Both DAIM and AVIMmonomers are able to polymerized by charges with good adhesion andcontact to the stainless steel electrode, while AMIM barely forms a filmon the surface likely due to limited unsaturated components on the ionicliquid. A uniform thin, membrane was found to cover the rough andscratched surface of the stainless steel electrode by polymerizing DAIMmonomer. The surface topography of the film formed by DAIM was furthercharacterized by atomic force microscopy (AFM) in tapping mode. A smoothand uniform height/topography image was seen from a DAIM prepared filmon stainless steel, which was comparable with an SEM image. To assessthe thickness of the film formed on the electrode, a scratch was made toexpose the stainless steel and allow measurement of the height from thestainless steel surface to the upper polymer, giving a thickness ofabout 80 nm.

The molecular weight (M_(w)) and polydispersity index (PDI) of theelectropolymerized IL films were determined by means of gel permeationchromatography (GPC) in dimethylformamide. It was seen that AMIM formsoligomers (Mw of about 896 g/mol) (possibly due to the limitedunsaturated double bonds), while DAIM and AVIM monomers were capable offorming large molecular weight polymer films (Mw of 58,540 g/mol and122,100 g/mol, respectively) consistent with the film morphologiescharacterized by SEM. The existence of sp² hybridized vinyl group inAVIM makes it more reactive and self-polymerizable in the presence aninitiating moiety, leading to much larger M_(w) and denser membranefilms than produced by electropolymerization of DAIM. Thus, whilemembranes formed by DAIM are rubber-like with softer consistency andmore open morphologies, those based on AVIM have molecular weight abouttwice as large and less open morphologies. Based on the morphology andstability of the film formed, DAIM was selected for further evaluation.To create membranes suitable for electrochemical studies, DAIM monomerwas used as additives in liquid electrolytes to form ionic membranes ina variety of configurations on sodium metal anodes. For brevity, we herefocus on membranes created in-situ on metallic sodium using 20 wt % ofthe IL monomer as an additive in an electrolyte comprised of 1 M sodiumperchlorate in ethylene carbonate/propylene carbonate (EC/PC—NaClO₄).

Sodium Metal Stability

To evaluate the stability of the sodium metal anodes passivated with ourionic polymer membrane SEI prepared with polymerizable ionomers, theCoulombic efficiency (CE) for sodium stripping and platting processeswas determined from galvanostatic experiments in a Na/stainless steelcell. Initially, a predetermined amount of sodium (1 mAh/cm²) was platedon the stainless steel electrode at a constant current of 1 mA/cm². Inthe following cycle, a fraction (⅙) of the sodium was striped and platedfrom the stainless steel repeatedly at the same current density. Underthese conditions, the CE can be calculated based on the simple formulaproposed by Aurbach (J. Electrochem. Soc. 1989, 136, 3198-3205). Controlcells that do not contain IL monomer in the electrolyte, or whichcontain the unpolymerizable IL monomer 1-methyl-3-propylimidizoliumchlorate (MPIM), were evaluated in the same manner and found to yieldvery low CE values around 16.7% and to fail within the first cycle forthe cells with no IL in the electrolyte. This result is expected as theunpassivated sodium metal is expected to vigorously and irreversiblyreact with liquid electrolyte during electrochemical cycling. The low CEis also consistent with previous reports in cycling experiments incarbonate electrolytes in which all of the plated sodium was strippedfrom the stainless steel electrode each cycle. Consistent with thediscussion in the introduction, the authors explain the low CE forsodium metal anodes in terms of formation of non-uniform solidelectrolyte interphase as well as dendritic growth.

In contrast, electrolytes containing DAIM as an additive exhibitmarkedly improved Coulombic efficiency with values as high as 95.0%measured at a current density of 1 mA/cm². Additionally, cellscontaining DAIM exhibit vastly improved stability in long-term cyclingmeasurements and lower overpotential compared with those containingMPIM. It is important here to note that while MPIM additives incarbonate electrolytes were reported previously to prevent dendriteformation in lithium metal batteries, the material only has a limitedeffect in improving the CE of the sodium metal anode, with only a 60% CEbeing achieved. As the surface of the neat stainless steel electrode isrelatively rough, this experiment tentatively confirms our previousfinding that DAIM monomer helps form a uniform ion conducting polymericmembrane that not only protects the sodium metal surface, butfacilitates transport at the anode.

A more detailed assessment of the membranes was performed using anaggressive galvanostatic polarization experiment in Na/Na symmetriccells in order to study the role played by the as prepared membranesover a wider range of current densities. FIG. 2 reports the celllifetimes at various current densities in the two cases, with andwithout DAIM additive in the electrolyte. It is seen that the celllifetime doubles at low current density and improves by at least afactor of three at higher current densities (>1 mA/cm²). As important isour observation that unlike typical lithium metal anodes, wherein cellfailure by internal short-circuit is evidenced by a large voltage dropduring galvanostatic polarization, failure of sodium anodes is almostalways evidenced by a diverging voltage, indicative of electrolytedegradation. Comparison of the voltage profiles at 0.1 mA/cm² shows ahigher polarization voltage with fluctuations for the cells in neatelectrolyte, indicative of severe side reaction between sodium andelectrolyte. The results reported here are to our knowledge the first toshow that sodium metal cells are more likely to fail by electrolytedepletion than by dendrite-induced short circuits. This observation isbelieved to derive from the relative softness of metallic sodium(room-temperature hardness: 0.5 MPa and shear modulus: 3.3 GPa) relativeto Li. It indicates that dendrite induced short-circuiting andassociated phenomena such as thermal runaway are not as important forsodium metal battery technology as they are for Li metal cells, andprovides insights into the role the IL membranes may play in stabilizingthe sodium anode.

The stability of the Na/Na symmetric cell containing DAIM monomeradditives in the electrolyte was also investigated by electrochemicalimpedance spectroscopy (EIS) experiments. To evaluate the interfacialreactivity of the sodium metal surface during polarization, impedance ofthe cells containing 0 or 20 wt % DAIM was measured as a function oftime following intervals of two-hour charging at 1 mA/cm². Theinterfacial resistance was found to be unstable for the control cell,displaying erratic values and transients from measurement tomeasurement. In contrast, in the 20 wt % DAIM case, the interfacialresistance steadily increases, indicative the formation of a stable SEIlayer by the ionic membrane. The electrochemical initiatedpolymerization process in the presence of sodium anode and electrolytewas further investigated by linear-sweep voltammetry. The oxidationcurrents after the breakdown voltage are seen to be much lower for ILcontaining electrolytes than for the control electrolyte, indicatingthat the IL containing electrolytes have better electrochemicalstability than the neat liquid electrolytes. It is also noted that smalloxidization peaks were observed between 2 to 4 V for the electrolytescontaining DAIM and AVIM, possibly indicative of anelectron-transfer-induced polymerization process. The peak currentincreases linearly with the increase of the root mean square of the scanrate, indicating a diffusion-limited controlled polymerization process.

Sodium Surface Characterization

To further verify that an ionic polymeric membrane prepared from ILmonomers helps stabilize sodium metal and prevent side reaction betweenelectrode and aprotic electrolyte, an in-situ optical microscopytechnique was applied to directly visualize how sodium deposits in aquartz cuvette optical Na/Na symmetric cell. A constant current of 1mA/cm² was applied to polarize one electrode and we choose this currentto make the study consistent with our previous test in coin cells. Lightmicroscopic images of the sodium electrode at different deposition timesin a 1M EC/PC—NaClO₄ electrolyte were obtained. Due to the softness ofsodium metal, the pristine sodium electrode surface was not perfectlysmooth. As the deposition process continued, the electrode was graduallycovered by two types of deposits-thin fiber-like and bulkymushroom-like, which were formed in a dynamically swinging movementmanner Both types of deposits grow not in a specific direction but sparkrandomly, which probably was due to the unevenness and defects of theelectrode surface. The shinning fiber-like deposit was discoveredpreviously on sodium metal surface at a very low current density of 57μA/cm² and on lithium electrodeposition as well. The bulky black sodiumdeposit was first discovered to the best of our knowledge, wetentatively attribute its formation to the uncontrolled side reactionwith electrolyte. This type of deposit grows faster and seems to be moredetrimental to the electrode as it quickly withdraws from the electrodesand floats randomly in the electrolyte, eventually becomes ‘dead sodium’which lowers the coulombic efficiency. When DAIM was added in theelectrolyte, a big improvement was observed during sodium deposition atthe same current density. It is apparent that the big blackmushroom-like deposit was completely eliminated during the whole processof deposition, which indicates that the in-situ formed ionic polymericmembrane protects sodium and eliminate the side reaction with liquidelectrolyte. Though the needle-like deposits still exist, it is seenthat the quantity of this type of dendrite is much smaller than that ofthe control and the growth rate is less than half than that of the bulkydendrites. The grows of the needle deposits may related to the effectivepore size (˜1 μm) of the polymer membrane, which was approximated fromthe storage modulus (4.05*10⁻³ Pa) of the polymeric DAIM. As theeffective pore is in the similar length scale to that of the needles andmuch smaller than mushroom deposits, it can successfully block thegrowing pathway of the mushroom deposits while some of the needles maypenetrate through the film and continue to grow.

As the condition and environment are quite different between coin cellsand optical cell due to assembling pressure and the existence ofseparators in the coin cells. We hypothesize with the help of mechanicalpressure and separator, sodium can be engineered into an effective anodeonce the side reaction is eliminated and sodium surface is protectedbased on a recent theory study on lithium dendrite growth. To verify ourbelieve, the morphology of the sodium metal anode after galvonastaticpolarization and after 10 cycles of stripping and platting in coin cellswere examined by SEM. As we expected, sodium anode polarized in theexistence of DAIM in electrolyte exhibits uniform and flat surfacemorphology with a film formed on top, indicative of dendrite freeelectrode deposition with the help of pressure and separator, however,the control sodium electrode was quickly oxidized when transferring itto the SEM sample holder. Though the MPIM and AVIM additives in theelectrolyte are beneficial compared with electrolyte with no such noadditives, they are not as effective as the DAIM additive which isprobably due to the non-contact protection of the sodium metal and thereactivity of the vinyl group. C 1s based on X-ray photoelectronspectroscopy (XPS) analysis on the control sodium metal and sodiumelectrode deposited in the prescience of DAIM in the electrolyteverifies the existence of IL components on the sodium surface afterone-minute sputtering on the sodium metal surface.

Electrochemical Performance for Sodium Storage

In order to evaluate the suitability of the protected sodium anode forroom-temperature sodium metal batteries, the sodium anode after 10hour's deposition in the presence of 20 wt % DAIM in the electrolyte inNa/Na symmetric cell was investigated under galvanostatic conditionswith Na₃V₂(PO₄)₃ and polyacrylonitrile-sulfur (SPAN) composite,respectively, as electrodes. Na₃V₂(PO₄)₃ is attractive because itexhibits high intercalation potential of around 3.4 V (vs Na⁺/Na), whichmakes Na₃V₂(PO₄)₃ a good candidate for next generation cathode materialsfor electrical energy storage, however, due to the larger ionic radiusof sodium, it is a challenge to allow stable Na ion extraction andinsertion. Previous work usually applied fluoroethylene carbonate (FEC)as electrolyte additive to stabilize the passivation layer betweenelectrode and electrolyte interphases, however FEC is known to decomposeand forms hazardous HF gas during electrochemical process. Therefore, itis urgent to develop alternative methods to stabilized this type ofcell. SPAN composite cathode was capable of delivering high capacity andcycle stability in lithium metal batteries, and it is compatible withcarbonate electrolyte. For these reasons, Na₃V₂(PO₄)₃ and SPAN serve asgood candidates for investigating the electrochemical stability of theprotected sodium anode in rechargeable batteries.

FIG. 3A reports the electrochemical characteristics of Na—Na₃V₂(PO₄)₃cells based on the DAIM treated Na metal anodes. A floating point testfrom 3 V to 5 V gives a time dependent current response for the cellwith protected sodium electrode up to 4.8 V, while an irregular andnoisy current response was observed for the control cell at almostentire voltage ranges. The improved cell stability over differentvoltage ranges directly relates to the reduced side reaction betweensodium anode and aprotic electrolyte. This enhanced cell stability alsoreduces the overpotential during galvanostatic cycling test, whichresults in higher cycling stability and coulombic efficiency (FIG. 3A).When the anode is protected by the ionic polymeric membrane, both thecapacity and efficiency rise, with the coulombic efficiency exceeding96% and capacity maintaining at 97 mAh/g after 160 cycles, while thecontrol cell exhibit a variation of coulombic efficiency and a quickdecrease of discharge capacity to below 20 mAh/g in 30 cycles. Webelieve this direct benefit of dramatic improvements stems from thetethered and uniform polymeric film that continuous supply ions near theelectrode and prevent sodium metal from reacting with liquidelectrolyte.

We also evaluate the application of the ion-rich sodium anode in Na-SPANbatteries. As SPAN has a high theoretical capacity, more coulombs willbe applied to the cell during cycling. Therefore, it is effective toevaluate the effect of the sodium anode protection in the system. As weexpected, the protected cell gives rise to an almost 100% coulombicefficiency and a reversible discharge capacity for over 100 cycles,however, the control cell bears random coulombic efficiency and thecapacity drops to 200 mAh/g after 100 cycles (FIG. 3B). The failure inthe Na-SPAN cell, more likely be the same with the Na—Na₃V₂(PO₄)₃ cell,could be attributed to the side reaction with the electrolyte duringcell charging, as the voltage profile is noisy and the efficiency isvery low. Those results clearly indicate that the importance ofprotecting sodium anode in sodium metal batteries and their widelyapplications.

In summary, we report an example of fabricating an ionic membrane frompolymerizable ionic liquids to protect metal anodes such as a sodiummetal anode. The protected anode can be cycled stably with highCoulombic efficiency at high current densities. The ionic membrane onthe anode was prepared by electroinitiated-polymerizing unsaturatedfunctional IL monomer in electrolyte in-situ. We use both spectroscopicand analytical tools to show that such polymeric IL films providesufficient amount of immobile ions on the electrode surface to preventsodium anode from reacting with aprotic electrolyte and reduce unevensodium electrodeposition. Electron and optical microscopy as well aselectrochemical analysis indicate that IL monomer form a protective filmon the Na anode and stabilize deposition of sodium by at least twomechanisms. First, they form an ionic polymeric SEI layer that protectssodium metal from parasitic side reactions with the liquid carbonateelectrolyte. Second, they appear to utilize a previously reportedtethered anion effect to stabilize deposition of Na. Our findingunderscores the benefits of protecting sodium metal anode in theapplication of inexpensive rechargeable sodium metal batteries bearinghigh voltage or high specific capacity.

Another aspect of the present disclosure includes ionic membranes thatform stable solid electrolyte interphases between a metal anode andelectrolyte based on a halogenated alkyl anion salt, e.g., a bromideionomer, which can adsorb onto the metal anode, e.g., a Li anode. Suchionic membranes can advantageously exhibit three attributes required forstable anode operation such as Li—O₂ cell operation.

First, they protect the Li anode against parasitic reactions and alsostabilize Li electrodeposition during cell recharge. Second, halogen(bromine) species liberated during the anchoring reaction function as aredox mediator for the recharge reaction at the cathode, reducing thecharge overpotential. Finally, such ionic membranes form anexceptionally stable interphase with Li, which is shown to protect themetal in high Gutmann donor number liquid electrolytes. Suchelectrolytes have been reported to exhibit rare stability againstnucleophilic attack by Li₂O₂ and other cathode reaction intermediates,but are known for their reactivity with Li metal anodes. The ionicmembrane design is able to regulate transport of matter and ions at theelectrolyte/anode interface and thus provide address major barriers topractical Li—O₂ storage technology.

In an embodiment of the present discloser, an ionic membrane is directlyformed on a metallic anode such a metallic lithium anode for arechargeable lithium-oxygen (Li—O₂) electrochemical cell. FIG. 4illustrates such an embodiment.

The rechargeable lithium-oxygen (Li—O₂) electrochemical cell is peerlessamong energy storage technologies for its high theoretical specificenergy (3500 Wh/kg), which far exceeds that of current state-of-the-artLi-ion battery technology. Li—O₂ cells are under intense study forapplications in electrified transportation because they are viewed asthe gateway to Li-air storage technology that is capable of offeringcompetitive specific storage capacities to fossil fuels. A Li—O₂ cellincludes a Li metal anode, an electrolyte that conducts Li⁺ ions, anduses O₂ gas hosted in a porous carbon or metal support as the activematerial in the positive electrode (cathode). The cell operates on theprinciple that Li₂O₂ is reversibly formed and decomposed in the cathode,with the net electrochemical reaction of 2(Li⁺+e⁻)+O₂Li₂O₂ at anequilibrium potential of 2.96 V versus Li/Li⁺.

The ionic membrane can be directly formed on a metallic anode byincluding an ionomer such as a halogenated alkyl sulfonate salt togetherwith a liquid electrolyte in the rechargeable battery. Such ahalogenated alkyl sulfonate salt can adsorb directly onto the anodesurface forming a tethered alkyl anion that supplies ions near the anodeelectrode and stabilize the anode metal electrode.

The additive and the in situ-formed SEI that it forms are deliberatelydesigned to take advantage of three fundamentally based mechanisms forstabilizing electrochemical processes at the anode and cathode of theLi—O₂ cell. First, consistent with predictions from recent continuum anddensity functional analyses of lithium deposition, we report thationomer electrolyte additives that can ensure low diffusion barriers andhigh cation fluxes in the SEI at the anode are highly effective instabilizing deposition of Li. We demonstrate the success of theseadditives by means of electrochemical analysis and postmortem imaging.Second, we show that if the ionomer additives are designed to form thinconformal coatings at the Li surface, it is possible to passivate theanode surface against chemical attack by high-DN (DN=27.8) liquidelectrolytes capable of stabilizing oxide intermediates on the cathode.Finally, we report that the same material that stabilizes Li depositionon the anode also functions as an effective redox mediator that lowersthe overpotential for the OER reaction at the Li—O₂ cathode.

Understanding the Anode Protection Mechanism

Characterization of the anode. The electrolyte ionomer salt additive(2-bromoethanesulfonate lithium salt) investigated for the presentembodiment can react with lithium as provided in the scheme below.

2Li+Br-CH₂CH₂—SO₃Li→LiBr+Li-CH₂CH₂—SO₃Li

Scheme 1. Shows the reaction of lithium 2-bromoethanesulfonate withlithium metal forming LiBr salt and lithium-based organometallic.

The particular ionomer was chosen for this embodiment because of itsability to react with lithium to simultaneously anchor lithiumethanesulfonate at the anode/electrolyte interface and to generatepartially soluble LiBr in the electrolyte. The specific ionomerchemistry selected for the study is motivated by four fundamentalconsiderations. First, recent continuum theoretical analysis andexperiments indicate that tethering anions, such as sulfonates at theanode/electrolyte interface, lowers the potential at the interfaceduring Li deposition and in so doing stabilizes the deposition. Second,joint density functional theoretical (JDFT) calculations show that theenergy barrier E_(a) for Li⁺ diffusion at a Li anode coated with LiBrsalt (E_(a,LiBr)≈0.03 eV) is much lower, by a factor of around 8,compared to Li₂CO₃(E_(a,Li2CO3)≈0.24 eV), which forms naturally whenaprotic solvents react with Li. This means that under isothermalconditions, stable deposition of Li in a given electrolyte can occur atdeposition rates more than three orders of magnitude higher on aLiBr-coated Li anode than on an anode with a spontaneously formedLi₂CO₃-rich SEI. Third, the short hydrocarbon stem that connects thetethered sulfonate groups to Li should allow a dense hydrocarbon brushto form at the interface to protect the Li electrode from chemicalattack by a high-DN electrolyte required for stability at the cathode.Finally, soluble LiBr undergoes electrochemical oxidation and reductionin an appropriate potential window to function as a soluble redoxmediator.

Cryo-focused ion beam (cryo-FIB) was used to characterize the morphologyand thickness of the ionomer-enriched electrode/electrolyte interfacewith the liquid electrolyte intact but cryo-immobilized. In thistechnique, a symmetric lithium cell (with an ionomer-based electrolyte)was opened manually, and the sample was snap-frozen by immediatelyplunging it into slush nitrogen to preserve the electrolyte and to avoidair exposure. The sample was then transferred under vacuum into an FEIStrata 400 FIB fitted with a Quorum PP3010T Cryo-FIB/SEM PreparationSystem and maintained at −165° C. for the duration of the experiment. Toproduce a cross section of the interface, we used the focused galliumion beam to mill through the frozen electrolyte and into the electrode.This interface was then examined by scanning electron microscopy (SEM)and energy-dispersive x-ray (EDX) spectroscopy directly in the cryo-FIB.SEM images revealed an interfacial layer up to approximately 25 nm thickin most areas. EDX analysis shows that the chemical composition of thelayer is similar to that in the bulk electrolyte and that brominespecies are distributed more or less uniformly throughout.

Additional insight into the nature of the interfacial region can beobtained by washing away the electrolyte and analyzing the ionomer layerthat remains immobilized on the Li metal. For this purpose, we used EDXand high-resolution x-ray photoelectron spectroscopy (XPS) analyticalmeasurements. The XPS measurements used monochromatic Al K-α x-rays(1489.6 eV) with a beam diameter of 1 mm to probe a surface layer on theelectrodes approximately 15 to 25 nm thick, that is, comparable to thethickness of the interface revealed by cryo-FIB. Sulfur and brominesignals are evident everywhere on the surface of the material. XPSanalysis was also performed using postmortem measurements on lithiumanodes harvested from Li—O₂ cells subjected to different runningconditions. High-resolution scans for anodes retrieved after cycling orafter a single discharge with the ionomer additive in the 1 MLiNO₃—N,N-dimethylacetamide (DMA) electrolyte were reviewed and comparedto a corresponding Li anode without the ionomer. From this comparison,it is apparent that after the first discharge, a Li 1s peak at 55.2 eVis observed on anodes with or without the ionomer present in theelectrolyte. The peak may be attributed to the presence of LiOH, Li₂O₂,and Li₂CO₃. A more prominent Li is peak is observed at 53.8 eV,accounting for about 85% of lithium, only in spectra of anodes cycled inthe presence of the ionomer additive. This peak is indicative of theformation of a different SEI in electrolytes containing the ionomer; Liis peaks with comparable binding energy are reported for organometallicscontaining Li—C bonds (54.2 eV). This observation is consistent with theionomer reacting at the Li anode surface to form a lithiumethanesulfonate-rich SEI at the interface. Also, the fact that thisbinding energy is observed in the cycled anodes confirms that the SEIlayer is stable and present even after repeated insertion and extractionof lithium ions into the underlying electrode.

Further evidence that the ionomer additive forms a stable SEI on Li canbe deduced from the 0 is and Br 3d high-resolution scans. The 0 is peakat 532.2 eV comprises approximately 18% of the oxygen signal in cellswithout the ionomer additive, whether the anodes originate from cellsthat were subjected to a single discharge or were cycled. The 532.2-eVpeak has been previously reported to originate from sulfonates, whichaccounts for 27 and 38%, respectively, of the oxygen signal when theanode is discharged once or cycled in the presence of the ionomeradditive. The corresponding sulfur atomic contribution for the samematerials can be computed from the wide survey scans to be about 2% forthe once discharged anode and about twice as high for the cycled anodes.The high-resolution scans of Br 3d reveal the formation of a single bond(a 3d_(5/2) and 3d_(3/2) doublet) with a Br 3d_(5/2) peak at 68.5 eVwhen the anode is discharged once in the presence of the ionomer. Weattribute this peak to the formation of the Br—Li bond, which has beenpreviously reported to occur at binding energies between 68.8 and 69.5.The same peak persists when the anode is cycled in the presence of theionomer, but with a contribution of only around 15%. The reduced Li—Brspecies in the anodes of cycled cells is an indication of LiBr beingsolvated by the DMA electrolyte that can further participate in theredox mediation of oxygen cathode recharging. A more prominent Br 3dpeak at 67.0 eV is observed only for the cycled anodes, likelyoriginating from Br—C bonds [binding energies between 66.7 and 71.0 eV]in the SEI originating from an untethered ionomer. The untetheredionomer in the electrolyte can help in the regeneration of the SEI layerin repeated cycling. Our results based on XPS analysis thus show thatthe ionomer-added electrolyte forms a SEI layer of lithiumethanesulfonate and LiBr, in accordance with the proposed reactionmechanism.

The effectiveness of ionomer-based SEI on Li was analyzed usingimpedance spectroscopy measurements on symmetric lithium cells. Theresults were compared with Nyquist-type plots at progressive timeperiods for control cells and those that contain 10% or 5% (by weight)ionomer additive. The experimental data points are fitted with thecircuit model to deduce the bulk and interfacial resistances (FIG. 5) asa function of time for the control electrolyte as well as with 10 and 5%(by weight) ionomer additive. It is seen that the bulk resistance forall cells remain essentially constant for approximately 20 hours, beyondwhich the bulk resistance of the control diverges (the increase is muchlarger for the results for the control cells after 48 and 56 hours). Thetime-dependent interfacial impedance provides an even more sensitiveindicator of the stability of the anode-electrolyte interphase in ahigh-DN solvent. It is seen that the initial interfacial resistances forcontrol and ionomer SEI-stabilized Li electrodes are approximately equal(˜50 ohms). However, there is an exponential rise in the interfacialresistance of the control cell over time consistent with rapid reactionbetween Li and DMA. It is important to note that this reaction isobserved although LiNO₃ is present at large concentrations in theelectrolyte. These results therefore challenge the view that LiNO₃provides an effective means of passivating Li metal anodes againstreactive liquid electrolytes. In contrast, the results in FIG. 5 showthat the interfacial resistance remains constant when the ionomer-basedSEI is present. It is seen that the stabilization with 10% ionomeradditive is marginally better than the 5% case. Together, these findingsdemonstrate that a SEI based on bromide ionomers has a large stabilizingeffect on Li anodes in DMA-based electrolyte solvents.

Lithium-electrolyte stability. The quality of lithium ion deposition onstainless steel substrates mediated by control and ionomer-containing 1M LiNO₃-DMA electrolytes were compared. For these experiments, cellswere assembled with lithium as an anode and stainless steel as a virtualcathode. Lithium with a capacity of 10 mAh/cm² was deposited at a rateof 1 mA/cm² onto stainless steel, after which the cell was rested for aperiod of 10 hours and the voltage was monitored over time. For the cellincluding an ionic membrane, 10 wt % of 2-bromoethanesulfonate lithiumsalt was added to the electrolyte to form the membrane. FIG. 6 showsthat in case of a control electrolyte, Li deposition takes place at ahigher voltage compared to the ionomer-containing electrolyte. Also, itcan be observed that after the rest period, the voltage measured in thecontrol cells immediately rises to approximately 0.5 V. This highopen-circuit potential after Li deposition is a reflection of thecomplete decomposition of Li deposits on stainless steel due tocorrosion by the electrolyte. It is again worth noting that despiteusing the Li-passivating salt LiNO₃ at high concentrations in theelectrolyte, the freshly deposited lithium reacts completely with theelectrolyte solvent. FIG. 6 also reports the corresponding voltageprofiles observed in rested cells containing the ionomer as anelectrolyte additive. It is seen that the cell voltage remains close to0 V (versus Li/Li⁺), that is, near the open-circuit potential of asymmetric lithium cell, which means that the Li electrode is chemicallystable in the reactive DMA electrolyte solvent.

To further examine the morphology of Li deposits, we performedpostmortem analysis, wherein the surface features of the electrodes werevisualized under a SEM. For the control, there are few patches of Liobserved, and large sections of bare stainless steel are clearlyvisible. In contrast, in electrolytes containing the ionomer, thestainless steel surface is covered with a thick layer of lithium. It isalso seen that Li electrodeposits formed in the latter electrolytes areevenly sized and spherical, even at a relatively high current density of1 mA/cm². This observation is consistent with previous reports of morecompact electrodeposition of Li in electrolytes with halidesalt-enriched SEIs and single-ion-conducting features.

To fundamentally understand the basis of these observations, wecharacterized the electrochemical stability of the electrolytes by meansof linear scan voltammetry in the range of −0.2 to 5 V versus Li/Li⁺, ata fixed scan rate of 1 mV/s. We plotted current as a function of voltagein a two-electrode setup of Li∥stainless steel. It is seen that for thecontrol, the current diverges at a value around 4 V versus Li/Li⁺,whereas for electrolytes containing ionomer additives, the currentdiverges at a higher voltage, around 4.3 V versus Li/Li⁺. This improvedstability is consistent with previous reports of electrolyte compositeswith tethered anions, wherein anions fixed at or near the electrodesurface limit access to and chemical reaction of anions in anelectrolyte with the negative electrode. Another important feature ofthe results can be seen at a potential close to 0 V versus Li/Li⁺. Thesignificant current peak apparent at approximately −0.2 V versus Li/Li⁺for both control and ionomer-containing electrolytes is a characteristicof lithium plating onto stainless steel. However, as the voltage isprogressively increased, the corresponding Li stripping peak is not seenin the control cell but is readily apparent in cells with theionomer-containing electrolyte. This behavior is indicative of thecomplete consumption of lithium deposits on stainless steel in thecontrol cells and is consistent with previous results of SEM.

Results from so-called galvanostatic “plating-stripping” experiment areused to evaluate the stability of Li electrodeposition and to assess thepropensity of the material to electrodeposit as rough, dendriticstructures. In contrast to previous studies, where thick (˜0.75 mm) Lifoil is used on both electrodes in plate-strip protocols, we performedthese experiments using asymmetric Li/Li cells composed of one thick Liand one Li-lean (10 mAh/cm² of Li deposited on stainless steel at 1mA/cm²) electrode. The stability of the Li deposition reaction isnormally assessed using three criteria: (i) magnitude of overpotentialof lithium deposition, (ii) steep decrease of the cell voltage to zerowith continuous charge-discharge, and (iii) a steady increase of thevoltage over extended cycles of charge and discharge. In the firstcriterion, higher overpotential is indicative of formation of insulatingproducts on the surface of the Li electrodes. At a fixed current density(0.05 mA/cm²), the voltage response for cells with ionomer-based SEI islow (approximately 6 mV), whereas the corresponding value for thecontrol is much higher (approximately 150 mV). The second criterion isrelated to the short-circuiting of the cell when dendritic lithium thatformed at one or both electrodes bridges the two electrodes. It isapparent that this phenomenon is not observed either in the control orfor the ionomer SEI-stabilized electrodes. Thirdly, a rise in voltageover cycles represents an unstable SEI that grows continuously,eventually consuming the Li deposited on the stainless steel substrate.It was observed, after only two cycles at both current densitiesstudied, the control cell fails after a steep rise in voltage. This isquite different from what is observed for cells in which Li isstabilized by an ionomer SEI, which is stable for over 150 cycles. FIG.7 reports the number of cycles at which the cell voltage diverges as afunction of current density (J). The ionomer-based SEI is seen toimprove cell lifetime at a fixed current density by nearly two orders ofmagnitude. These results underscore the effectiveness of theionomer-based SEI in stabilizing electrodeposition of Li in amide-basedelectrolytes, which were previously thought to be unfeasible for lithiummetal batteries because of their high reactivity with and readydecomposition by Li.

Anode protection mechanism. We hypothesize that the stability of the Lianode in DMA originates from two fundamental sources: (i) accumulationof LiBr salt at the Li/electrolyte interface, which facilitates Li-iontransport to the Li electrode during charging; and (ii) the existence oftethered sulfonate anions at the interface, which lowers the electricfield at the electrode. Previous JDFT analysis revealed that thepresence of lithium halides in the SEI of Li metal anode lowers theactivation energy barrier by an order of magnitude or more for lateralLi diffusion at a Li/electrolyte interface, thereby increasing thetendency of Li to form smooth deposits. Comparing the surface diffusionbarriers for various constituents of a typical SEI layer, it wasreported that Li₂CO₃, a common SEI constituent in carbonateelectrolytes, has an energy barrier of 0.23 eV, whereas the barrier fora SEI composed of LiF is 0.17 eV. This difference has been arguedpreviously to explain the much greater tendency of Li to form flat,compact deposits during battery recharge, as revealed by experiments inwhich weakly soluble LiF salts are enriched in the SEI by precipitatingout of liquid electrolytes. The JDFT analysis shows that the activationenergy barrier for Li-ion diffusion at a LiBr/Li interface is much lower(0.062 eV) and comparable to that of magnesium, which is known in theliterature to electrodeposit without formation of dendrites. Thus, theLiBr created during the formation of the SEI should provide an even morepowerful (than LiF) stabilizing effect on Li deposition.

In addition to the presence of LiBr, the SEI created by the ionomercontains bound anionic groups in the form of lithium ethanesulfonate(Li—CH₂CH₂—SO₃ ⁻). Thus, the electrolyte includes a combination of freeand tethered anions. In the past, researchers have realized theimportance of single-ion-conducting electrolytes, because theseelectrolytes prevent the formation of ion concentration regions within acell, leading to stable ion transport even at a high charge rate. Recentlinear stability analysis of electrodeposition showed that the stabilityof an electrolyte can be significantly enhanced by immobilizing only asmall fraction (10%) of the anions. The design of an electrolyte forthis embodiment, which is composed of a fraction of anions near theanodic surface, with LiNO₃ as the free salt, is to capture thistheoretical framework. Thus, a modified SEI based on bromide ionomerstethered to the Li anode provides a powerful combination of processesthat stabilize the anode against unstable electrodeposition.

Characterizing cathode products. A representative voltage profile forthe galvanostatic discharge and charge for a Li—O₂ cell with 1 M LiNO₃in an ionomer-enriched DMA electrolyte was prepared. Cutoff voltages of2.2 and 4.3 V were used for the discharge and charge cycles,respectively, and both processes were performed at a fixed currentdensity of 31.25 μA/cm². Postmortem SEM analysis was used to study theevolution of discharge products on the cathode at three stages ofdischarge (D1, D2, and D3) and two stages of charge (C1 and C2). The SEMimages show the reversible formation and decomposition of an insolublesolid product on the cathode. Complementary x-ray diffraction (XRD)analysis shows that the cathode product is exclusively Li₂O₂ (no otherproducts, such as LiOH, are observed). The SEM analysis shows that Li₂O₂particles grow increasingly larger as the discharge progresses andnucleation sites for growth are filled, and the full discharge capacityof the cell is reached. Analysis of the particle sizes on dischargereveals that, at low current densities (for example, 15 μA/cm²), largeLi₂O₂ particles (1 μm and higher) are formed. Comparing these results tothose reported by Lau and Archer (Nano Lett. 15, 5995-6002 (2015)) forLi—O₂ cells discharged in a 1 M LiTF in tetraethylene glycol dimethylether (TEGDME) (a low-donor number solvent), the Li₂O₂ particles formedin DMA are at least four times larger. These findings are consistentwith expectations for the high DN of DMA, which solvates Li⁺ cations andenables a solution-mediated mechanism, circumventing capacitylimitations from the passivation layer formed at the cathode, whichenables deep discharge. At higher current densities, the particle sizeat the voltage cutoff decreases drastically, consistent with the ideathat kinetic diffusion limitations set the maximum particle size. Uponcharge, the SEM images show a cathode that closely resembles that of thepristine electrode before discharge. Redox mediation from lithium2-bromoethanesulfonate is thought to aid in the electrochemicaldecomposition of the large, insulating Li₂O₂ particles formed on thecathode. Support for this belief comes from the effectiveness of therecharge process as well as from the flat charge profile observed untilthe full capacity of the discharge is reached; the voltage ultimatelybegins to rise because of the set voltage limit of 4.3 V. Thus, it wasshown that a Li—O₂ cell with 1 M LiNO₃-DMA in an ionomer-based SEI on Lican reach a high capacity through LiO₂ disproportionation, fully use theformed Li₂O₂ during the recharge, and cycles with features indicative ofthe presence of a redox mediator.

Cycling performance. To evaluate whether a high-DN electrolyte solventand a redox mediator provide significant synergistic benefits for Li—O₂cells, we compare the voltage profiles for fully discharged cellswithout and with these attributes. It is seen that the dischargecapacity of Li—O₂ cells with a 1 M LiNO₃-DMA with an electrolytecontaining ionomer is noticeably higher (˜6.5 mAh) than the dischargecapacity of Li—O₂ cells with a conventional 1 M lithiumbis(trifluoromethane)sulfonimide (LiTFSI)-diglyme (˜5.1 mAh) with thesame cathode loading. This finding is consistent with the observation oflarge-sized lithium peroxide structures owing to the solution-mediatednucleation of peroxides. Comparison of the charge cycle shows that withthe diglyme electrolyte, the voltage diverges to >4.2 V in ˜3.5-mAhcapacity, which is believed to be an indication of Li₂CO₃ formation andits effect on the charging process, whereas with the ionomer-basedelectrolyte, the voltage diverges at ˜6.5 mAh (same as discharge).Cyclic voltammetry experiment for a Li—O₂ cell in a two-electrode setupwith lithium as both reference and counter electrode were performedbetween 1.9 and 4.5 V (versus Li/Li⁺) at a scan rate of 1 mV/s, and thenormalized current is plotted against voltage. The current peaks for theionomer-based electrolyte are an order of magnitude higher than thosefor the control electrolyte. Thus, it can be inferred that there ishigher electrochemical activity owing to the higher stability of theelectrolyte and redox mediation due to the presence of LiBr. The peakseen at ˜3.5 V can be attributed to a Br₃ ⁻/Br⁻ redox couple.

Discharge and charge profiles for cells having the electrolyte 1 MLiNO₃-DMA with and without ionomers with a capacity cutoff of 3000 mAh/gand a current density of 0.04 mA/cm² were prepared. It was seen thatboth discharge and charge voltage curves tend to diverge to lower andhigher values, respectively. Further, it was seen that the voltageprofile becomes extremely noisy in the fifth cycle of the controlelectrolyte, whereas that with the ionomer additive is stable. Thisinstability without ionomers can be attributed to the degradation of theelectrolyte by reaction with the unprotected lithium metal. One majorbenefit of cells cycled with ionomers is reduced overpotential duringcharge relative to that of the control cell, thus increasing cyclingefficiency. This is studied in a Li—O₂ battery with a lower capacitycutoff of 800 mAh/g at a current density of 0.08 mA/cm² for the controlelectrolyte and the ionomer-added electrolyte. The highest voltage oncharge for cells with ionomers is approximately 3.7 V, close to theBr⁻/Br₃ ⁻ redox reaction at 3.48 V. Control cells with solely 1 MLiNO₃-DMA reach voltages of around 4.45 V. This suggests a similaraction to a redox mediator, in which Li₂O₂ is oxidized by Br₃ ⁻ toreform Br⁻ in a cycle that lowers charge overpotential. The dischargeand charge profiles remain similar over 30 cycles for cells withadditive, whereas the charge profile in untreated cells increases moredrastically. The distinct gentle slope of the initial portion of thedischarge profile in cells with ionomers can be attributed to thepresence of bromine species in the system. FIG. 8 compares the endvoltage of recharge with and without the ionomer additive. The ˜1-Vimprovement in the round-trip efficiency not only saves loss of inputenergy but also ensures long-life cycling by preventing electrolytedecomposition.

Cathode stabilization mechanism. At the cathode surface, LiBr is thoughtto participate in the redox mediation that promotes the OER reaction. Inthis process, the Li₂O₂ can be co-reduced with Br⁻ to form O₂ and Br₃ ⁻.The potential for Br⁻→Br₃ ⁻ is 3.48 V; thus, the charging of a Li—O₂cell can be limited to this voltage. DMA's ability to dissolve peroxidesalso aids in the effective electrolyte-side redox mediation. Support forthe uniqueness of these ideas comes from recent experiments thatdemonstrate the efficacy of LiI and LiBr as redox mediators in Li—O₂cells based on glymes. In the absence of water in the electrolyte, LiIwas reported to produce a gradual rise in the discharge voltage due toformation of iodine and similar products. LiBr was found to beineffective in maintaining a steady charge voltage. In electrolytes withhigh water content and LiI, LiOH has been shown to be the primarydischarge product, which has been reported to be thermodynamicallyimpossible to undergo OER. Our results therefore clearly show thatprotecting the Li anode in a 1 M LiNO₃-DMA electrolyte with a SEI basedon bromide ionomer overcomes fundamental limitations of the anode,cathode, and electrolyte in previously studied systems and enablesstable cycling of these cells.

In summary, the present disclosure demonstrates that the addition of anionomer to an electrolyte, viz lithium 2-bromoethanesulfonate (ionomer)to 1 M LiNO₃-DMA electrolytes, produces an ionic membrane that act likea SEI at the lithium surface that stabilizes the anode in Li—O₂ cells byat least two powerful processes. Compared to control cells with theionomer SEI, Li—O₂ cells based on lithium 2-bromoethanesulfonate exhibitflatter, more stable charge profiles and can withstand deeper cycling.Furthermore, we show that electrochemical charge-discharge processes inthe cells coincide with the formation and decomposition of large Li₂O₂particles as the principal OER product in the cathode. Analysis bylinear scan voltammetry and “plate-strip” cycling analysis of the Lianode show that a SEI based on a lithium 2-bromoethanesulfonate ionomeron the anode provides chemical stability to Li against attack by DMA, aswell as physical stability against rough, dendritic electrodeposition.

Examples

The following examples are intended to further illustrate certainpreferred embodiments of the invention and are not limiting in nature.Those skilled in the art will recognize, or be able to ascertain, usingno more than routine experimentation, numerous equivalents to thespecific substances and procedures described herein.

Materials Synthesis: Functional IL monomers AMIM, DAIM and AVIM bearingperchlorate anion were synthesized according to the previous methodswith modifications [7a]. In this work, NaClO4 was used as the anionexchange source. The Na3V2(PO4)3 [23a] and SPAN [25b] cathode wassynthesized based on the previous studies. The synthesized Na3V2(PO4)3contains no graphene additive.

Material Characterization: The morphology of the sodium metals wasstudied by A LEO 1550 high resolution scanning electron microscopy(SEM), Olympus light microscope and Asylum MFP-3D atomic forcemicroscopy(AFM). Waters Ambient-Temperature GPC was applied to analyzethe molecular weight of the polymers. X-ray photoelectron spectroscopy(XPS) measurements were performed with a Surface Science SSX-100spectrometer using a monochromatic Al Kα source (1486.6 eV). Non-linearleast squares curve fitting was applied to the high-resolution spectra,using CasaXPS software.

Electrochemical Characterization: Protected sodium metal anode wascreated by charging Na/Na symmetric cell at constant current of 1 mA/cm2for 10 hours in a carbonate electrolyte containing 20 wt % IL monomers.2032 coin-type cells were assembled using sodium metal (Sigma Aldrich)as the anode electrode, a microporous material, Celgard 3401, membranesas separator, a cathode with 80% active material, 10% Super-P Li carbonblack from TIMCAL, and 10% poly (vinylidene difluoride) (PVDF, SigmaAldrich) as binder in an excess of Nmethyl-2-pyrrolidone in NMP, andelectrolyte of 50 uL 1M sodium perchlorate (NaClO4) in ethylenecarbonate/propylene carbonate (EC/PC vol:vol=1:1) for each cell. Cellassembly was carried out in an argon-filled glove-box (MBraunLabmaster). The room-temperature cycling characteristics of the cellswere evaluated under galvanostatic conditions using Neware CT-3008battery testers and electrochemical processes in the cells were studiedby cyclic voltammetry using a CHI600D potentiostat. Electrochemicalimpedance and floating tests were conducted by using a Solartron CellTest System model 1470E potentiostat/galvanostat. For post-mortemstudies, cells were disassembled in an argon-filled glove-box and theelectrodes were harvested and rinsed thoroughly with the electrolytesolvent before analysis.

Li—O₂ Battery Methods and Materials

Cathode preparation. A cathode slurry was prepared by mixing 180 mg ofSuper P carbon (TIMCAL), 20 mg of polyvinylidene fluoride(Sigma-Aldrich), and 2000 mg of N-methyl-2-pyrrolidone (Sigma-Aldrich)in a ball mill at 50 Hz for 1 hour. Toray TGP-H-030 carbon paper wascoated with an 80-μm-thick layer of carbon slurry using a doctor blade.The resulting coated carbon paper was dried at 100° C. overnight undervacuum and transferred into an argon-filled glove box [O₂, <0.2 partsper million (ppm); H₂O, <1.0 ppm; Innovative Technology] withoutexposure to air. Disks (15.9 mm in diameter) were punched and weighedfrom the carbon paper to yield individual carbon cathodes. The weight ofthe active carbon layer (not including the carbon paper) averaged1.0±0.1 mg.

Electrolyte preparation. LiNO₃ and LiTFSI were heated under vacuumovernight at 100° C. to remove all traces of water and transferreddirectly into the glove box. DMA (Sigma-Aldrich) and bis(2-methoxyethyl)ether (diglyme; Sigma-Aldrich) solvents were dried over 3 Å molecularsieves (Sigma-Aldrich). Lithium 2-bromoethanesulfonate was obtainedthrough ion exchange with sodium 2-bromoethanesulfonate (Sigma-Aldrich).

Coin cell assembly. First, a 0.5-inch-diameter (12.7-mm-diameter) holewas punched in the top (cathode) side of each CR2032 case. Then, astainless steel wire cloth disk [disk diameter, 0.75 inches (19 mm);wire diameter, 0.0055 inches (0.140 mm)] from McMaster-Carr was added,followed by a cathode disk, a 19-mm-diameter separator (either WhatmanGF/D glass fiber or Celgard 3501), 100 μl of desired electrolyte,0.5-inch-diameter lithium metal, a 15.5-mm-diameter stainless steelspacer disk, a stainless steel wave spring (MTI Corporation), and ananode cap of the CR2032 case. The assembly was crimped to a pressure of14 MPa with a hydraulic coin cell crimple (BT Innovations).

Testing environment. Cells were tested at a regulated pure O₂environment of 1.3 atm and allowed to equilibrate for 6 hours beforeelectrochemical testing. Galvanostatic measurements were conducted usinga Neware CT-3008 battery tester.

Cyclic voltammetry. The cyclic voltammetry test was performed in atwo-electrode setup of Li∥air cathode. The batteries were cycled between1.9 and 4.5 V at a scan rate of 1 mV/s several times.

Evaluating Anode Stability

Impedance spectroscopy. Cells in the symmetric configuration wereassembled in an Ar glove box. Measurements were carried out using aSolatron frequency analyzer at a frequency range of 10⁻³ to 10⁷ Hz. Thedata were fitted into Nyquist-type plots using the equivalent circuitshown in FIG. S2 with the software ZSimpWin. Impedance was conducted atroom temperature at various time intervals.

Linear scan voltammetry. Linear scan voltammetry was performed in aLi∥stainless steel cell. The batteries were first swept to −0.2 V versusLi/Li⁺ and then they were swept in reverse direction until the voltagediverges.

Lithium versus stainless steel cycling. For cycling tests, lithiumversus stainless steel cells were prepared and were cycled at 0.01mA/cm² between 0 and 0.5 V 10 times to form a stable SEI layer. Then,different tests were carried out as previously described.

Characterization Techniques

SEM and energy-dispersive analysis of x-rays. Discharged cells weredisassembled inside the glove box, and the cathodes were removed andtransported to the SEM (Zeiss, LEO 1550 Field Emission SEM) within anairtight container. The cathodes were loaded onto the stage in thepresence of a nitrogen stream. Images were taken with a single passafter focusing on a nearby region. Energy-dispersive analysis of x-ray(EDAX) measurements were performed by taking multiple counts on a smallsection of the sample.

X-ray diffraction. Cathodes were mounted on a glass microscope slideinside an argon-filled glove box and coated with paraffin oil to protectthem from air during the XRD measurements. Measurements were performedon a Scintag Theta-Theta x-ray diffractometer using Cu K-α radiation atλ=1.5406 Å and fitted with a two-dimensional detector. Frames werecaptured with an exposure time of 10 min, after which they wereintegrated along χ (the polar angle orthogonal to 20 to yield anintensity-versus-20 plot.

X-ray photoelectron spectroscopy. XPS was conducted using SurfaceScience Instruments SSX-100 with an operating pressure of ˜2×torr.Monochromatic Al K-α x-rays (1486.6 eV) with a beam diameter of 1 mmwere used. Photoelectrons were collected at an emission angle of 55°. Ahemispherical analyzer determined electron kinetic energy using a passenergy of 150 V for wide survey scans and 50 V for high-resolutionscans. Samples were ion-etched using 4-kV Ar ions, which were rasteredover an area of 2.25 mm×4 mm with a total ion beam current of 2 mA, toremove adventitious carbon. Spectra were referenced to adventitious C isat 284.5 eV. CasaXPS software was used for XPS data analysis with Shelbybackgrounds. Li 1s and O 1s were assigned to single peaks for each bond,whereas Br 3d was assigned to double peaks (3d₅₁₂ and 3d₃₁₂) for eachbond with 1.05-eV separation. Residual SD was maintained close to 1.0for the calculated fits. Samples were exposed to air only during theshort transfer time to the XPS chamber (less than 5 s).

While the claimed invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to one ofordinary skill in the art that various changes and modifications can bemade to the claimed invention without departing from the spirit andscope thereof. Thus, for example, those skilled in the art willrecognize, or be able to ascertain, using no more than routineexperimentation, numerous equivalents to the specific substances andprocedures described herein. Such equivalents are considered to bewithin the scope of this invention, and are covered by the followingclaims.

What is claimed is:
 1. A rechargeable battery comprising a metal anode,a cathode and an electrolyte, wherein the metal anode has an ionicmembrane thereon which can continuously supply ions near the metal anodeand protect the metal anode.
 2. The rechargeable battery of claim 1,wherein the ionic membrane comprises an ionic polymer membrane.
 3. Therechargeable battery of claim 2, wherein the ionic polymer membrane isformed from one or more polymerizable ionic liquids having more than onepolymerizable groups and an anion.
 4. The rechargeable battery of claim3, wherein the metal anode comprises substantially metallic sodium. 5.The rechargeable battery of claim 1, wherein the ionic membranecomprises alkyl anions tethered to the metal anode.
 6. The rechargeablebattery of claim 5, wherein the alkyl anions tethered to the metal anodeare formed from one or more halogenated alkyl anion salts.
 7. Therechargeable battery of claim 5, wherein the alkyl anions tethered tothe metal anode are formed from one or more halogenated alkyl sulfonatesalts.
 8. The rechargeable battery of claim 7, wherein the metal anodecomprises substantially metallic lithium.
 9. The rechargeable battery ofany one of claims 1-8, wherein the electrolyte includes one or moreionomers that form the ionic membrane.
 10. The rechargeable battery ofany one of claims 1-9, wherein the rechargeable battery has a coulombicefficiency exceeding 90%.
 11. The rechargeable battery of any one ofclaims 1-10, wherein the rechargeable battery can stably cycle over 100cycles.
 12. The rechargeable battery of any one of claims 1-11, whereinthe ionic membrane has a thickness in the range of about 5 nm to about500 microns.
 13. The rechargeable battery of any one of claim 1-3, 5-7,or 9-12 wherein the metal anode comprises substantially metalliclithium, metallic sodium, metallic aluminum, or metallic zinc.
 14. Amethod of preparing a metal anode electrode for a rechargeable battery,the method comprising: forming a conformal ionic membrane directly on asurface of the metal anode from one or more ionomers wherein the ionicmembrane can continuously supply ions near the metal anode and protectthe metal anode.
 15. The method of claim 14, wherein forming theconformal ionic membrane includes electropolymerizing a polymerizableionic liquid monomer as the ionomer onto the metal anode to form theionic membrane.
 16. The method of claim 14, comprising forming theconformal ionic membrane directly on the surface of a substantiallymetallic sodium anode by electropolymerizing a polymerizable ionicliquid monomer having more than one polymerizable groups onto the metalanode to form the ionic membrane.
 17. The method of claim 14, whereinforming the ionic membrane includes adsorbing halogenated alkyl anionsalts as the ionomer onto the metal anode to form the ionic membrane.18. The method of claim 14, wherein forming the ionic membrane includesadsorbing halogenated alkyl sulfonate salt as the ionomer onto asubstantially metallic lithium anode as the metal anode to form theionic membrane.
 19. The method of any one of claims 14-18, wherein theionomer is added to an electrolyte of the rechargeable battery.
 20. Themethod of claim 19, wherein the ionomer is added to the electrolyte inan amount of from about 1 to about 30 weight percent of the electrolyte.